HOW FLOWERS ATTRACT INSECTS 



209 



that were uninjured, it follows that the brightly coloured corolla exerts an attrac- 

 tive influence. 



It was only in the case of Antirrhinum majus that the mutilated flowers received 

 no visitors. After Plateau had cut down a number of such flowers till they were 

 only I cm. long, the humble-bees (Bombus terrester Z.) which were sucking the 

 uninjured blossoms showed no inclination to visit them. Plateau tries to explain 

 this by saying that in order to reach the nectar of the mutilated flowers — which are 

 open above — by flying up from below, the insects could get no foothold except by 

 suddenly converting their ascending movements into descending ones. For this 

 reason they preferred to forgo the use of the nectar in the mutilated flowers, and to 

 seek out uninjured blossoms in the neighbourhood. 



This explanation does not seem to me to be a happy one, because these insects 

 settle on many other flowers from above, without suffering any inconvenience from 

 this mode of alighting. It appears to me far more probable that ike odour 0/ Anti- 

 rrhinum majus is not sufficient /or purposes of attraction, but that here the form and 

 colour of the corolla play a leading part in this matter, so that the insects do not visit 

 mutilated flowers in which these means of allurement are wanting. 



Plateau conducted a most interesting experiment with Centaurea Cyanus. He 

 removed the blue ray-florets — which according to our view merely serve to attract 

 and are therefore neuter — leaving only 

 the rather inconspicuous disk-florets 

 {see Fig. 81). In spite of this many 

 bees (Megachile ericetorum) visited the 

 cornflowers so treated. Plateau sup- 

 poses that the insects were attracted 

 by the odour. I believe that this 

 occurrence is quite as easily explained 

 by assuming that the visitors of intact 

 cornflower capitula have become ac- 

 quainted with the form of the nectar- 

 yielding disk-florets, and are able — from 

 a short distance — to recognize them 

 after the removal of the ray-florets just 

 as well as before. 



Friedrich Dahl, among others, has 

 proved that insects are able to distinguish forms accurately at a short distance 

 (cf. Zool. Anz., Leipzig, xxii, 1889). Professor Dahl told me of the following 

 observation, which can only be explained on the assumption that insects are 

 guided by the sense of sight.— The young honey-bees that appear in spring 

 settle upon flowers with nectar beyond their reach owing to the depth at which 

 it lies, and make vain attempts to suck it. Older bees (that have already 

 swarmed), on the other hand, come near such flowers, but turn away without 

 settling on them. This must be due to the fact that the older bees having learnt 

 that the nectar of these flowers is not within their reach, look at them on getting 

 near and realize that there is nothing to be had. Were it the specific odour 

 of the flowers that deterred them from visiting they would not need to go so 



Fig. 81. Centaurea Cyanus^ L. (after 

 Platwiu). a, Capitulum with ray-florets ; 

 5, capitulum without ray-florets. 



